Substrates used in the electronics, optics, and optoelectronics fields are often fabricated from a starting material that is obtained on an industrial scale in the form of ingots. Various ingot fabrication methods (e.g., ingot pulling methods) are known including the Czochralski pulling method (CZ pulling) and the zone melting method (FZ pulling). These methods produce various shapes of ingots of unrefined material, in particular ingots in the shape of a cylinder having two ends that are substantially conical. A silicon ingot may be approximately 1 m to 2 m long.
Such ingots are then cut to eliminate pointed ends, their central cylindrical portion segmented into a plurality of segments, and each segment sliced into a plurality of wafers. Each wafer is finished by, e.g., grinding and polishing, so that its two opposite faces are flat, and then by, e.g., chemical etching steps, so that dust, residual particles, and zones damaged during the preceding material-removal steps are eliminated. Finished wafers are referred to herein “bulk virgin substrates” or “bulk silicon substrates”. For example, a 300 mm diameter ingot is generally cut into 20 cm to 30 cm segments and approximately 200 bulk virgin substrates are obtained from each segment, while a 200 mm diameter ingot is cut into 30 cm to 40 cm segments and 200 or more bulk virgin substrates are obtained from each segment.
Each bulk virgin substrate may then be used for further fabricate of, e.g., silicon on insulator (“SOI”) wafers. Generally, a SOI type substrate is fabricated by splitting a thin surface layer off a bulk silicon substrate, termed the “donor”, and transferring the split layer onto a substrate, termed the “receiver”, after interposing a layer of silicon oxide. After transferring a first layer, a donor substrate can be recycled, i.e., a further thin layer is split therefrom and transferred to a second receiver substrate. Such a substrate is termed a “recycled bulk substrate” in the remainder of the description and claims. A donor substrate may be recycled three or four times.
The quality of the transferred layers, and of the final hybrid substrates from which electronic components are fabricated, depends essentially on the quality of bulk substrates. However, bulk substrate often have crystalline defects, which, depending on their number and location, can render it unsuitable for subsequent fabrication. Crystalline defects include: dislocations and voids, which may be isolated or form clusters; agglomerations of interstitial or void type point defects; stacking defects; oxygen precipitates; amorphous or crystalline inclusions of foreign phases (e.g., suicides); metallic precipitates; crystal originated particles (COP); and so forth. COP type defects present in the bulk substrate appear on the transferred layers.
Methods have been developed with the aim of reducing or eliminating defects. See, e.g., EP-A-1 158 581. CZ ingot pulling methods have also been improved to avoid COPs or other types of crystalline defects. U.S. publication 2005/0064632 describes a technique, termed the “copper deposition method”, for making defects more apparent in order to characterize and for select wafers suitable for the SOI fabrication. That technique consists in forming a film of oxide at a predetermined depth from the surface of a wafer, removing a portion of the oxide, depositing a solution of copper electrolytes onto the wafer, and applying an electric current. Cu ions then precipitate in regions where the oxide layer is degraded, said regions corresponding to defect zones in the underlying wafer.
Even when using methods to fabricate improved ingots and methods for limiting defects, however, it has been observed that defects still exist and in a latent state that cannot be revealed by current techniques. Especially important is an ability to detect latent crystalline defects which subsequently appear after recycling a wafer several times. There is an economic advantage of being able to predict the quality of bulk substrates before using them in the production line, especially of being able to predict defects which appear only after recycling several times.